PROJECT SUMMARY The gut epithelial cell layer and the mucus it secretes provide a barrier between host tissues and foreign luminal contents. Compromised barrier function induced by environmental triggers and/or genetic factors is associated with inflammatory bowel disease (IBD), which affects approximately 1.6 million Americans each year and increases the risk of developing colorectal cancer. Although intestinal homeostasis is crucial for human health, we do not fully understand the underlying processes that stabilize the barrier, or how to therapeutically correct deficiencies in its function. Intestinal epithelial homeostasis is controlled by cytokines secreted by innate lymphoid cells including Lymphoid Tissue inducer (LTi) cells. These cells exert broad affects on intestinal physiology and immunity, due to their production of the cytokines IL-22 and IL-17A, which directly modulate epithelial cells. The relevance of these cytokines is apparent in IL-22- and IL-17A-deficient mice, which exhibit increased susceptibility to bacterial infection. Soluble mediators that induce cytokine production by LTi have been identified, but the field lacks a molecular paradigm for LTi inhibition. My preliminary studies indicate that the cell surface receptor RANK suppresses cytokine production in LTi. How RANKL suppresses LTi, and its relevance during various disease states is unclear. I will test the hypothesis that dual expression of RANK and RANKL on neighboring LTi cells activates TRAF6 and prevents their hyper- responsiveness during homeostasis and infection. AIM1 will determine whether direct cell-cell interactions limit LTi effector cytokine production in cryptopatches. AIM2 will identify the molecular pathways that suppress LTi during intestinal inflammation. I will also use an in vivo genetic screen to establish whether RANK signaling is the predominant mechanism of LTi suppression, or whether there are additional novel inhibitory factors. These studies will reveal molecular details about a new axis of innate lymphoid cell regulation and advance our ability to target LTi and other innate lymphocytes to improve human health.